Due to advances in semiconductors and related technologies, light-emitting diodes (LEDs) have become so cost-effective as to make them feasible for lighting systems that previously relied upon incandescent or discharge lamps. Consequently, a substantial variety of LED-based replacement solutions have become available.
In the realm of household lighting, replacement of incandescent bulbs with bulbs that utilize LEDs has become commonplace. LED-based bulbs are still more costly than standard incandescent bulbs, but offer certain advantages, such as improved energy efficiency and much greater operating life.
In the realm of industrial lighting (e.g., factories and warehouses) and area lighting (e.g., office spaces and large residential spaces), the transition from conventional light sources to LED-based light sources has likewise proceeded at a fast pace. One of the major challenges has been the fact that many of those environments include large numbers of lighting fixtures which already include ballasts (magnetic and/or electronic) that are specifically designed for powering discharge lamps.
In recent years, many efforts have been directed to the challenge of providing LED-based light sources that are so-called “drop in” replacements for existing discharge lamps. These “drop-in” replacement LED-based light sources are commonly housed within a package resembling that of a conventional discharge lamp tube, which is typically a linear tube with mercury or gas inside. The solid-state replacement typically includes a number of LEDs (arranged in various series, or series-parallel, combinations), along with associated circuitry, to functionally take the place of the discharge lamp(s) that they replace. Description of such “drop-in” replacements may be found in U.S. Pat. No. 9,713,236 and U.S. patent application Ser. No. 14/644,111 (published as 2015/0260384, which have the same assignee as the present application.
In the conventional discharge lamp tube each end has two pins are connected to a filament between them. The result is a pair of pins and filament at each end of the lamp. Typical lamp lengths are 2-foot, 3-foot, 4-foot and 8-foot lengths although other sizes are available for special applications. The lamps with two pins at each end are known as bipin lamps.
Ballast are traditionally needed to drive these conventional lamps. The ballast can be low frequency magnetic that operate at 60 Hz or a high frequency ballast that converts the main voltage, 120 Vac at 60 Hz, to a high frequency AC sinusoidal waveforms at the proper voltage to drive the lamps. Typically, high frequency is 20 Khz to 65 Khz.
The conventional, discharge lamps operate by containing a gas within the tube, which ionizes when sufficient voltage is provided across the pins at the ends. The excitation of the gas results in the release of energy that causes the phosphor coating on the interior of the tube to glow, thus providing light. As described above, LED replacement lamps typically use a string of light emitting diodes to functionally replace the gas filled tube.
A traditional fluorescent lamp for example is non-conductive until the voltage between the two filaments is great enough to ionize the gas in the lamp and cause it's impedance to drop and conduct current. This current causes light in the lamp. The ionization voltage varies with the heating of the two filaments at each end of the lamp. By applying a small AC voltage across each filament, current flow heats the filaments and lowers the ionization voltage.
Both magnetic and high-frequency ballasts are designed to keep the voltage across the lamp or lamps less than ionization level until the filaments are heated. The voltage required to ionize the lamp reduces as the filaments are sufficiently heated.
High frequency ballasts are isolated from the main voltage and ground by an isolation transformer as part of the high-frequency inverter. Magnetic ballasts, however, are simple non-isolated autotransformers that have voltage potential relative to safety ground. When replacing lamp with the ballast energized there is a potential shock hazard between the bipins of the lamp and the safety grounded fixture. This can happen when only one end of the lamp is inserted in to the lamp holder.
Safety standards have been developed fluorescent lamp ballasts, including standard UL935. UL935 specifically includes a standard test for current shock and has a test for lamps when one end is inserted into an energized ballast. UL limits are 5 millamps rms or 7.07 millamps peak, when voltage applied to the inputs is 170 Vac rms, or less. Recently, UL modified the standard to include LED replacement lamps that are being used with existing conventional ballasts intended for use with fluorescent lamps. The voltage at which current may flow in some LED replacement lamps may be much lower than that for a fluorescent lamp, e.g. 70 V to 90 V for a 4 foot lamp.
When LED lamps are used on high frequency electronics, the UL935 test is readily met. However, magnetic ballast output leads are not DC isolated from the mains voltage and safety ground. Any resulting voltage which may exist in a magnetic ballast is insufficient to ionize the gas and cause conduction above the test limits for a conventional discharge lamp. However, LED lamps have much lower conduction voltage than a traditional fluorescent lamp and conduction can occur, and the UL935 test failed, for voltages exceeding the conduction voltage of the LED lamp. The need exists, therefore, for the design of circuitry which may be included in an LED drop-in replacement lamp which more closely emulates the behavior of a traditional fluorescent lamp and satisfies the safety requirements of UL935.